Domain-Driven Design SWEN-261 Introduction to Software Engineering - PowerPoint PPT Presentation

Domain-Driven Design SWEN-261 Introduction to Software Engineering Department of Software Engineering Rochester Institute of Technology

Domain driven design centers the architecture on the problem domain.  Quote from the DDD Community: Domain-driven design (DDD) is an approach to developing software for complex needs by deeply connecting the implementation to an evolving model of the core business concepts  The premise: • Place the project’s primary focus on the core domain and domain logic • Base complex designs on a model • Initiate a creative collaboration between technical and domain experts to iteratively cut ever closer to the conceptual heart of the problem 2

Let's review our project architecture. Client UI Server UI Model Application Network Connection What goes in these two tiers? HTML, CSS Spark & Frameworks & JavaScript FreeMarker Any Java Browser Platform Web server (Jetty) Any Any OS and HW OS/HW OS/Hardware 3

DDD provides guidance for the remaining tiers. Client UI Server UI Model Application Network Connection HTML, CSS Spark & Frameworks & JavaScript FreeMarker Any Java Browser Platform Web server (Jetty) Any Any OS and HW OS/HW OS/Hardware 4

Services provide application logic.  The Application tier is responsible for managing the user's interaction with the application.  It is not responsible for domain logic which is in the Model layer.  Application tier elements provide services to each client connection. • Manage application-wide logic and information • Provide client-specific services for the UI tier 5

Entities provide domain logic.  The Model tier is responsible for managing domain entities and domain logic.  Entity responsibilities are: • Process user requests/commands • Effect changes based on user requests/commands • Validate Model-tier rules • Maintain the state of the Model  Entities often represent information about the world, and are inspired by domain model entities • Customers, products and orders in e-commerce • Shapes in a drawing app 6

Value objects provide values for an entity's "complex" attributes.  A value object class encapsulates the data that represents an entity's attribute. • Measurements, dates, credit card numbers, money, colors, (x,y) coordinates are some examples. • Two value objects are equal based on equality of the data in the object not object identity.  Value objects must be immutable. • An address of 15 N. Main St cannot be changed into 352 2 nd Ave. • You create a new address object of 352 2 nd Ave.  A value object class is not just a data holder class. • Value Object = "value semantics" + immutability + GRASP Information Expert, ref. Flight class in OOD I 7

Model objects are frequently used in collections.  Many of the algorithms used in Model and Application components require using Entities and Value Objects in hash-based collections.  Normal Java equality semantics are not adequate when dealing with Entities and VOs • An Entity must have a distinct id such that two objects with the same id must be considered equal. • Two Value Objects with the same data must be equal. • These semantic requirements imply specialized equals and hashCode methods.  The after-class exercise provides instructions on how to create these methods. 8

A semantically correct value object can be used as a key in a map collection.  Rather than extracting attributes from the value object to create a key, use the value object directly.  This will work correctly because • The value object is immutable  other code with a reference to the object can not change the object's value while it is in the map as a key • The equals and hashCode methods ensure that two objects with the same value will be considered equal and generate the same hash code. 9

Let's review the architecture again. 10

This is the list of component responsibilities. UI Tier Application Tier Model Tier UI Controller: Service: Entity: • Control the views based on the state of the • Manage application-wide • Process user requests/commands • application logic and information Effect changes to the Model based • Query the Model and Application tier as • Provide client-specific on user requests/commands • necessary to get information to present to the services to the UI tier Validate model rules • user Maintain the state of the model • Perform simple input validation and data conversion based on input modality, e.g. Value Object: String to integer • Initiate processing of user • Provide immutable value requests/commands possibly providing data semantics • the user input Provide value-based logic • Perform data conversion for display by views UI View: • Provide an interface to the user • Present information to the user in a variety of ways • Provide a mechanism for user to input data and requests 11